Diffusion bonding is a solid state process relying on atomic migration across an interface. The bond is formed during the application of temperature and pressure over a period of time. The two surfaces to be joined require a good surface finish (Ra <0.4µm) and must be clean. As a general rule, diffusion bonding is undertaken at a temperature of 80% of the melting point of the material.

Ceramic-Ceramic

As with most diffusion bonding scenarios, the flatter (and more parallel) the surfaces, the less time will be required at a given temperature for bonding to occur.

General engineering ceramics have been successfully joined to themselves, to make components that would otherwise have been too difficult or expensive to produce by other means.

Ceramics containing 'glassy' intergranular phases (such as alumina and silicon nitride) have been particularly successful. Success can be further assured by utilising a thin layer of the appropriate glass between the pieces to be joined. Slightly more surprising is the success achieved with materials such as silicon carbide and zirconia.

Microwave Diffusion bonding

An interesting variation on diffusion bonding of ceramics to them selves is to use microwave radiation as the heating sources.

Work in the 1990s showed that engineering ceramics could be diffusion bonded, using a 2.45 GHz microwave source. With 94% alumina (Al₂O₃) was particularly successful; showing high bend strength, with failure occurring in the parent material.

Diffusion bonding of ceramic to metal

The technique can be applied to a wide range of materials including metals, ceramics and combinations thereof. (If dissimilar materials are used then 80% of the lowest melting point material is used.)

In many instances direct bonding is not possible due to thermal expansion mismatch or metallurgical incompatibility. It is then that an interlayer or combination of interlayers is used. An interlayer thickness of a few microns is commonly used and this can be pre-placed as a thin foil interlayer or by sputter coating.

In addition, TWI can offer the following facilities:

Joint design
Interlayer design
Materials selection
Sputter coating
Process selection
Joint analysis by Scanning electron microscopy (SEM), optical microscopy Energy dispersive X-ray (EDX), mechanical testing and non-destructive testing (NDT)
Pre-surface assessment can also be made using surface measurement equipment.

Further Information

Best practice guides: Joining ceramics - a guide to best practice
Knowledge Summaries: Diffusion Bonding; Diffusion bonding - ceramics and ceramic/metal joints
FAQs: How do you diffusion bond ceramic to ceramic; Can you diffusion bond ceramics using microwaves
Staff papers: Aluminium metal matrix composites - successes using diffusion bonding Industrial Members Only; Improved facility for high temperature diffusion bonding; Diffusion bonding of titanium alloys; Techniques for bonding ceramics (Bulletin, September/October 1990) Industrial Members Only

For more information please contact: ceramics@twi.co.uk

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